A world that slumbers beneath Kusoka, Phantasmagoria is filled with Shadows, and must be explored to progress the story. There are two parts of Phantasmagoria- Story Dungeons, and Hikari's Dreamscape.
Story Dungeons
As time goes on, new locations will become available in Phantasmagoria. These locations must be completed as quickly as possible, as there is always a deadline attached. However, if a location is fully explored, there are rewards for doing so.
Some Story Dungeons may not be fully explored before their deadlines, but until a certain point in the story is reached, they may be revisited after completion, though the usefulness of the rewards gained for doing so may no longer be worth it past a given point.
Story Dungeons will, generally, contain Shadows of up to three Arcana, though some have fewer. They also only have one mandatory boss fight attached, though later ones may have... additional encounters.
Hikari's Dreamscape
Easily the largest accessible location in Phantasmagoria, exploration of Hikari's Dreamscape can provide power ups for various party members, progression in the story, and an altogether wider range of options as progress is made. This Dreamscape is even less linear than the others, though progress in certain areas can be gated off by lack of progress in other areas, or not having the right party member yet.
There are multiple bosses contained within this dreamscape, and various... additional encounters, that can be triggered or avoided based on player decisions. Shadows of every kind can appear here.
Hikari's Dreamscape is inaccessible while there is a Story Dungeon awaiting completion. Full exploration of Hikari's Dreamscape is highly unlikely- it's uncertain if it's even
possible- but there are definitely powerful rewards for those who would attempt to do so.
Progression
Progression in Phantasmagoria is divided into sections. Exploring a given section takes up one time period, and multiple time periods in a day may be devoted to exploration, as many as there is time free. As actions taken after leaving Phantasmagoria carry penalties, it is recommended that exploration that starts earlier on in the day carry on through evening, though only one dreamscape may be visited in a day.
Each section has a particular goal that must be completed to clear it, whether it's the party defeating a certain number of Shadows, or solving a given number of puzzles, or simply managing to traverse the hostile terrain filled with... additional encounters. Depending in the approach taken for going about this, the exact results may change.
In general, to complete an area, not every section will have to be cleared, though, of course, there are rewards for if it happens. If there isn't enough time before a deadline, these locations can generally be revisited, up to a certain point.
Do Not Exhaust Your Friends
If Phantasmagoria is visited for more than two full days in a row, your party members will Not be Happy with you. They will also start complaining after five days of visits that
don't take up the full day.
Be good to Hikari's friends. They're probably the only thing keeping her from dying.